Sorry, my mistake I should have said it was engaging and slowly turning the engine over, just not enough to start it. Looks like I may have caused it myself by keeping it turning over for too long, although I have definitely turned it over for longer before without this happening.

The slower it's turning, the more current it draws. Most starter motors are unusual in the way they are wound compared to 'normal' DC motors; the field windings are in series with the armature windings. This has the effect of producing a very powerful magnetic field since there is very big current draw at the start of cranking when the motor is at its slowest and, consequently, a very high starting torque. For info, a motor wound this way will spin in the same direction if its polarity is reversed, unlike the more usual DC motor.

Perhaps stating the obvious but if the wiring damage is localised e.g. only in the vicinity of the alternator or starter it is more likely to be a short circuit within the starter motor windings or the solenoid. However it would be prudent to check the alternator just to make sure its not seizing up as this could overtly overload the starter (slip the belt of and see if it turns freely by hand) it is also worth checking the all the connections; especially the negative (earth). A lot of folk concentrate on the positive however if the earth is poor the system will look for an alternative and if this is a lighter wire then under load it can cause a rise in temperature and eventually fire. Im not sure of your location but if you not a million miles away I would be happy to turn up with the multi meter and high discharge tester and take a look at the complete circuit right back to the ignition switch!.

I'd put the non starting down to the low voltage ,and the wires melting down to starter motor being too hot due to continued turning over .

The voltage thing I have had on my opti . Smartcraft says one thing , but the engine only gets a few V due to corrosion in the wires/switch etc ( mine had 5v despite the smartcraft saying 12.2v) - have a look.. its amazing how quickly things can corrode .. mine went from 100% reliable starting to 50% in about a month.

The corrosion was just too much to give enough V to start the ECU and turn the engine over as has been said.

The other option is the starting relay - on the stb side of the engine at the bottom there are a few in a row .. I was told all identical ( but dont take my word - look at the writing on the side) so you can swap them to see if its this. The others are for the trim motor...

Had the alternator and starter checked by a specialist and turns out rectifier on alternator failed and caused it to arc, resulting in my melted cables. I was advised that a new loom may be the best option as a few cables in the loom were also melted!!!

Batteries themselves won't cause any problems with melted wires providing they are the same voltage and you simply swapped the two over. If you connected the negative to positive and visa versa then I would expect there to be built in protection to the outboard to protect it's self. So that just leaves the starter motor and alternator.

You said you tried to turn it over for 30 seconds. Was the starter not cranking the engine?

If this was indeed the case then it could be either a case of seized engine or faulty starter, both conditions that could draw more current from the battery (starter motor under heavy load or shorted) and so heat up the "weak link" in the wiring and melt it. There are not normally in line fuses on such power cables.

I have a couple of Opti's and had a similar problem with a 200hp opti sport jet motor. It had been flooded with water, which is a whole nother story. However, the wires were smoking and the engine was barely turning over. I used a wrench on the flywheel (Not sure if that is the correct term but the geared wheel at the top of the engine) and cranked it over manually a few times to purge the water. I poured some fuel down the intake and cranked it over. It caughed and sputtered but it started and eventually ran just like new. Those Opti's can take some abuse!

I would take the air cleaner off and look down the manifold for water or fuel. If you manually crank it a few times it should clear anything that is preventing it from starting.